开源框架源码解析系列（1）——进入OkHttp的世界

老马的编程之旅

发布于 2022-06-22 13:20:01

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发布于 2022-06-22 13:20:01

以前曾经写过一篇关于Okhttp的使用的文章深入解析OkHttp3，通过这篇文章可以了解OkHttp的各种基本用法，光会使用并不算好汉，我们还要深入理解源码，学习优秀的设计思想，本篇我就带大家一起分析源码，基于Okhttp 3.10.0版本。

1.请求部分源码解析

1.1 回顾请求的基本用法

1.1.1 发送同步请求

 Request request = new Request.Builder().url(url).build();
        try {
            //同步请求
            Call call = mOkHttpClient.newCall(request);
            Response response = call.execute();
            String json = response.body().string();
            Log.d(TAG, json);

        } catch (IOException e) {
            e.printStackTrace();
        }

1.1.2 发送异步请求

 //异步请求
        Call call = mOkHttpClient.newCall(request);
        call.enqueue(new Callback() {
            @Override
            public void onFailure(Call call, IOException e) {
                Log.d(TAG, "onFailure:" + e.getMessage());
            }

            @Override
            public void onResponse(Call call, Response response) throws IOException {
                String json = response.body().string();
                Log.d(TAG, json);
            }
        });

1.2 同步请求源码分析

首先，我们要理解，无论是同步请求还是异步请求，我们都需要先编写以下代码：

OkHttpClient mOkHttpClient = new OkHttpClient();
Request request = new Request.Builder().url(url).build();
Call call = mOkHttpClient.newCall(request);

Call是请求的关键对象，是通过调用Call的execute方法之后，就会进入请求的逻辑

Response response = call.execute();

1.2.1 OkHttpClient# newCall

@Override public Call newCall(Request request) {
    return RealCall.newRealCall(this, request, false /* for web socket */);
  }

我们可以看到OkHttpClient# newCall会调用RealCall.newRealCall方法

1.2.2 RealCall # newRealCall

 static RealCall newRealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) {
    // Safely publish the Call instance to the EventListener.
    RealCall call = new RealCall(client, originalRequest, forWebSocket);
    call.eventListener = client.eventListenerFactory().create(call);
    return call;
  }

查看一下构造函数

  private RealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) {
    this.client = client;
    this.originalRequest = originalRequest;
    this.forWebSocket = forWebSocket;
    this.retryAndFollowUpInterceptor = new RetryAndFollowUpInterceptor(client, forWebSocket);
  }

RealCall的创建过程中会持有OkHttpClient，请求的 Request还有创建了一个拦截器RetryAndFollowUpInterceptor（这个后面会详细说明），同时创建eventListener 。

1.2.3 Call# execute()

创建好Call之后，调用execute()方法就开始了请求的流程，Call是一个借口，所以我们要查看它的实现类RealCall。

1.2.4 RealCall# execute()

@Override public Response execute() throws IOException {
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    captureCallStackTrace();
    eventListener.callStart(this);
    try {
      client.dispatcher().executed(this);
      Response result = getResponseWithInterceptorChain();
      if (result == null) throw new IOException("Canceled");
      return result;
    } catch (IOException e) {
      eventListener.callFailed(this, e);
      throw e;
    } finally {
      client.dispatcher().finished(this);
    }
  }

这里重点在于如下两句

 client.dispatcher().executed(this);
 Response result = getResponseWithInterceptorChain();

调用OkHttpClient持有的Dispatcher对象执行call，Dispatcher是非常重要的一环，后面详细介绍。

1.2.5 Dispatcher# execute()

 synchronized void executed(RealCall call) {
    runningSyncCalls.add(call);
  }

private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();

将同步请求的RealCall 添加到同步的队列中。

1.2.6 RealCall# getResponseWithInterceptorChain()

通过RealCall# getResponseWithInterceptorChain()方法就可以获取请求返回的Response，返回到调用者

Response getResponseWithInterceptorChain() throws IOException {
    // Build a full stack of interceptors.
    List<Interceptor> interceptors = new ArrayList<>();
    interceptors.addAll(client.interceptors());
    interceptors.add(retryAndFollowUpInterceptor);
    interceptors.add(new BridgeInterceptor(client.cookieJar()));
    interceptors.add(new CacheInterceptor(client.internalCache()));
    interceptors.add(new ConnectInterceptor(client));
    if (!forWebSocket) {
      interceptors.addAll(client.networkInterceptors());
    }
    interceptors.add(new CallServerInterceptor(forWebSocket));

    Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0,
        originalRequest, this, eventListener, client.connectTimeoutMillis(),
        client.readTimeoutMillis(), client.writeTimeoutMillis());

    return chain.proceed(originalRequest);
  }

这里主要是一系列拦截器的添加操作，然后调用Interceptor.Chain的proceed方法去执行请求

chain.proceed(originalRequest)

拦截器又是另一个非常重要的环节，后面重点提到。

1.2.7总结同步请求

同步请求逻辑相对简单，通过Call# execute()最终会调用 RealCall# execute()，然后通过分发器Dispatcher将任务添加到同步队列中，然后通过一系列拦截器操作后进行请求，最后返回Response，全程都在主线程中运行，是阻塞式的。

1.3 异步请求源码分析

1.3.1 Call#enqueue

异步请求，会调用Call#enqueue方法，因为是异步，所以需要传递一个Callback回调，Call#enqueue中调用了RealCall#enqueue

1.3.2 RealCall#enqueue

@Override public void enqueue(Callback responseCallback) {
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    captureCallStackTrace();
    eventListener.callStart(this);
    client.dispatcher().enqueue(new AsyncCall(responseCallback));
  }

RealCall#enqueue中同样会调用到Dispatcher中，只是调用enqueue方法，同时new AsyncCall将Callback 包一层.AsyncCall是RealCall的内部类，从中可以获取RealCall的Request等成员。

1.3.2 Dispatcher#enqueue

 synchronized void enqueue(AsyncCall call) {
    if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
      runningAsyncCalls.add(call);
      executorService().execute(call);
    } else {
      readyAsyncCalls.add(call);
    }
  }

  /** Running asynchronous calls. Includes canceled calls that haven't finished yet. */
  private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();

  /** Running synchronous calls. Includes canceled calls that haven't finished yet. */
  private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();

主要做了几件事：

1.判断正在执行的异步任务队列中任务数是否小于maxRequests，且正在执行的任务的host小于 maxRequestsPerHost，这两个值的大小为：

  private int maxRequests = 64;
  private int maxRequestsPerHost = 5;

同时满足条件，则将AsyncCall 添加到runningAsyncCalls队列中，runningAsyncCalls是异步任务的队列，否则添加到readyAsyncCalls等待队列中。

2.调用Dispatcher#executorService方法，获取Android系统提供的线程池

  public synchronized ExecutorService executorService() {
    if (executorService == null) {
      executorService = new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60, TimeUnit.SECONDS,
          new SynchronousQueue<Runnable>(), Util.threadFactory("OkHttp Dispatcher", false));
    }
    return executorService;
  }

**注意：**这里设置线程池的最大容量为 Integer.MAX_VALUE，但其实受限于maxRequests，所以最多容量也就64而已。

3.通过ExecutorService执行AsyncCall任务，可想而知AsyncCall一定是实现了Runnable接口。

1.3.3 NamedRunnable#run

AsyncCall继承自NamedRunnable，所以当AsyncCall任务执行时，会执行NamedRunnable#run

 @Override public final void run() {
    String oldName = Thread.currentThread().getName();
    Thread.currentThread().setName(name);
    try {
      execute();
    } finally {
      Thread.currentThread().setName(oldName);
    }
  }

主要逻辑在execute()方法

1.3.4 AsyncCall#execute

@Override protected void execute() {
      boolean signalledCallback = false;
      try {
        Response response = getResponseWithInterceptorChain();
        if (retryAndFollowUpInterceptor.isCanceled()) {
          signalledCallback = true;
          responseCallback.onFailure(RealCall.this, new IOException("Canceled"));
        } else {
          signalledCallback = true;
          responseCallback.onResponse(RealCall.this, response);
        }
      } catch (IOException e) {
        if (signalledCallback) {
          // Do not signal the callback twice!
          Platform.get().log(INFO, "Callback failure for " + toLoggableString(), e);
        } else {
          eventListener.callFailed(RealCall.this, e);
          responseCallback.onFailure(RealCall.this, e);
        }
      } finally {
        client.dispatcher().finished(this);
      }
    }

可以看到还是会和同步请求一样，调用getResponseWithInterceptorChain()执行各种拦截器，返回Response，无论是同步还是异步最后都会执行Dispatcher#inished()方法，这个后面会提到。

1.3.5 总结异步请求

Call#enqueue会调用到 Dispatcher#enqueue，然后判断是否符合最大请求数maxRequests（64），最大请求Host数maxRequestsPerHost （5），符合条件的添加到异步任务队列runningAsyncCalls，通过线程池执行任务，否则添加到等待队列readyAsyncCalls。

2 Dispatcher分析

前面分析同步和异步请求的时候，都提到Dispatcher，我们这里重新总结一下：

1.维护了3个队列，同步请求执行队列runningSyncCalls，异步请求执行队列runningAsyncCalls，异步请求等待队列readyAsyncCalls，3个队列的添加逻辑前面已经提过。

 /** Ready async calls in the order they'll be run. */
  private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>();

  /** Running asynchronous calls. Includes canceled calls that haven't finished yet. */
  private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();

  /** Running synchronous calls. Includes canceled calls that haven't finished yet. */
  private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();

2.维护了异步请求的线程池，异步执行任务通过线程池进行任务执行

  public synchronized ExecutorService executorService() {
    if (executorService == null) {
      executorService = new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60, TimeUnit.SECONDS,
          new SynchronousQueue<Runnable>(), Util.threadFactory("OkHttp Dispatcher", false));
    }
    return executorService;
  }

3.RealCall中请求任务执行完后，进行回收，我们知道Call会被封装成RealCall，但无论同步还是异步执行完成后，都会调用以下代码

finally {
        client.dispatcher().finished(this);
      }

2.1 Dispatcher#finished

分为同步和异步的finished，我们先看同步的代码：

2.1.1 同步finished

 /** Used by {@code Call#execute} to signal completion. */
  void finished(RealCall call) {
    finished(runningSyncCalls, call, false);
  }

注意这里传入的第三个参数为false

private <T> void finished(Deque<T> calls, T call, boolean promoteCalls) {
    int runningCallsCount;
    Runnable idleCallback;
    synchronized (this) {
      if (!calls.remove(call)) throw new AssertionError("Call wasn't in-flight!");
      if (promoteCalls) promoteCalls();
      runningCallsCount = runningCallsCount();
      idleCallback = this.idleCallback;
    }

    if (runningCallsCount == 0 && idleCallback != null) {
      idleCallback.run();
    }
  }

1.首先将队列中call进行移除

2.如果promoteCalls为true还会调用 promoteCalls()方法

3.计算runningCallsCount，即为同步和异步执行队列的size总和

public synchronized int runningCallsCount() {
    return runningAsyncCalls.size() + runningSyncCalls.size();
  }

4.当runningCallsCount为0时说明已经没有任务了，进行回调

 if (runningCallsCount == 0 && idleCallback != null) {
      idleCallback.run();
    }

 void finished(AsyncCall call) {
    finished(runningAsyncCalls, call, true);
  }

2.1.2 异步finished

异步代码如下：

 void finished(AsyncCall call) {
    finished(runningAsyncCalls, call, true);
  }

和同步的区别是，传入的promoteCalls为true，所以当执行finished时会比同步多执行一个promoteCalls()方法

Dispatcher#promoteCalls

  private void promoteCalls() {
    if (runningAsyncCalls.size() >= maxRequests) return; // Already running max capacity.
    if (readyAsyncCalls.isEmpty()) return; // No ready calls to promote.

    for (Iterator<AsyncCall> i = readyAsyncCalls.iterator(); i.hasNext(); ) {
      AsyncCall call = i.next();

      if (runningCallsForHost(call) < maxRequestsPerHost) {
        i.remove();
        runningAsyncCalls.add(call);
        executorService().execute(call);
      }

      if (runningAsyncCalls.size() >= maxRequests) return; // Reached max capacity.
    }
  }

逻辑很清晰，就是当异步执行队列readyAsyncCalls有空闲位置时，遍历等待队列，将readyAsyncCalls的任务取出加入readyAsyncCalls，然后线程池对任务进行执行。

3.拦截器解析

3.1 拦截器执行顺序

在之前分析同步和异步任务的时候，分析过getResponseWithInterceptorChain()方法执行后就会返回请求结果Response

 Response getResponseWithInterceptorChain() throws IOException {
    // Build a full stack of interceptors.
    List<Interceptor> interceptors = new ArrayList<>();
    interceptors.addAll(client.interceptors());
    interceptors.add(retryAndFollowUpInterceptor);
    interceptors.add(new BridgeInterceptor(client.cookieJar()));
    interceptors.add(new CacheInterceptor(client.internalCache()));
    interceptors.add(new ConnectInterceptor(client));
    if (!forWebSocket) {
      interceptors.addAll(client.networkInterceptors());
    }
    interceptors.add(new CallServerInterceptor(forWebSocket));

    Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0,
        originalRequest, this, eventListener, client.connectTimeoutMillis(),
        client.readTimeoutMillis(), client.writeTimeoutMillis());

    return chain.proceed(originalRequest);
  }

1.首先将调用者自定义的拦截器都放入interceptors集合的最前面，然后是分别添加okhttp中必须的几个拦截器，后面我们会一一分析

2.创建拦截器的链RealInterceptorChain,将interceptors传入

从上述代码可以看出拦截器调用的先后顺序依次是

client.interceptors()–>RetryAndFollowUpInterceptor–>BridgeInterceptor–>CacheInterceptor–>ConnectInterceptor–>client.networkInterceptors()–>CallServerInterceptor

这里使用到了非常经典的设计模式，就是责任链模式，reques自上而下下传递执行，然后Response至下而上返回

这里3个参数是为 null的

3.2 RealInterceptorChain#proceed

@Override public Response proceed(Request request) throws IOException {
    return proceed(request, streamAllocation, httpCodec, connection);
  }

关键部分代码如下：

   // Call the next interceptor in the chain.
    RealInterceptorChain next = new RealInterceptorChain(interceptors, streamAllocation, httpCodec,
        connection, index + 1, request, call, eventListener, connectTimeout, readTimeout,
        writeTimeout);
    Interceptor interceptor = interceptors.get(index);
    Response response = interceptor.intercept(next);

又创建了一个RealInterceptorChain，然后获取interceptors中第一位的拦截器开始执行，这里index=0。然后就会按照顺序执行各拦截器。

3.3 RetryAndFollowUpInterceptor

如果没有自定义拦截器情况下，首先会走到RetryAndFollowUpInterceptor的intercept方法

@Override public Response intercept(Chain chain) throws IOException {
    Request request = chain.request();
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    Call call = realChain.call();
    EventListener eventListener = realChain.eventListener();

    StreamAllocation streamAllocation = new StreamAllocation(client.connectionPool(),
        createAddress(request.url()), call, eventListener, callStackTrace);
    this.streamAllocation = streamAllocation;

    int followUpCount = 0;
    Response priorResponse = null;
    while (true) {
      if (canceled) {
        streamAllocation.release();
        throw new IOException("Canceled");
      }

      Response response;
      boolean releaseConnection = true;
      try {
        response = realChain.proceed(request, streamAllocation, null, null);
        releaseConnection = false;
      } catch (RouteException e) {
       	//省略
      } catch (IOException e) {
      	//省略
      } finally {
        // We're throwing an unchecked exception. Release any resources.
        if (releaseConnection) {
          streamAllocation.streamFailed(null);
          streamAllocation.release();
        }
      }

      // Attach the prior response if it exists. Such responses never have a body.
      if (priorResponse != null) {
        response = response.newBuilder()
            .priorResponse(priorResponse.newBuilder()
                    .body(null)
                    .build())
            .build();
      }

      Request followUp = followUpRequest(response, streamAllocation.route());

      if (followUp == null) {
        if (!forWebSocket) {
          streamAllocation.release();
        }
        return response;
      }

      closeQuietly(response.body());

      if (++followUpCount > MAX_FOLLOW_UPS) {
        streamAllocation.release();
        throw new ProtocolException("Too many follow-up requests: " + followUpCount);
      }

      if (followUp.body() instanceof UnrepeatableRequestBody) {
        streamAllocation.release();
        throw new HttpRetryException("Cannot retry streamed HTTP body", response.code());
      }

      if (!sameConnection(response, followUp.url())) {
        streamAllocation.release();
        streamAllocation = new StreamAllocation(client.connectionPool(),
            createAddress(followUp.url()), call, eventListener, callStackTrace);
        this.streamAllocation = streamAllocation;
      } else if (streamAllocation.codec() != null) {
        throw new IllegalStateException("Closing the body of " + response
            + " didn't close its backing stream. Bad interceptor?");
      }

      request = followUp;
      priorResponse = response;
    }
  }

1.创建StreamAllocation

StreamAllocation streamAllocation = new StreamAllocation(client.connectionPool(),
        createAddress(request.url()), call, eventListener, callStackTrace);
    this.streamAllocation = streamAllocation;

主要传入OkHttpClient中的ConnectionPool，还有通过请求request.url()创建出Address对象，主要是HTTP请求中一些SSLSocket，host认证，Dns等

realChain.proceed

  try {
        response = realChain.proceed(request, streamAllocation, null, null);
        releaseConnection = false;
      }

realChain执行proceed方法，此时streamAllocation已经有值传入，此时再次进入RealInterceptorChain#proceed方法中

public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
      RealConnection connection) throws IOException {
    if (index >= interceptors.size()) throw new AssertionError();

    calls++;

 	//省略
    // Call the next interceptor in the chain.
    RealInterceptorChain next = new RealInterceptorChain(interceptors, streamAllocation, httpCodec,
        connection, index + 1, request, call, eventListener, connectTimeout, readTimeout,
        writeTimeout);
    Interceptor interceptor = interceptors.get(index);
    Response response = interceptor.intercept(next);

  	//省略
    return response;
  }
}

这里再次进来index已经变为1，然后又再新创建一个RealInterceptorChain，从interceptors中取出下一个拦截器，执行下一个拦截器的逻辑

**总结一下：**这里责任链模式，每一个拦截器执行时都会创建一个拦截器链RealInterceptorChain，index也会随之增加1，这样在 interceptors.get(index)中就会取出下一个拦截器，一直向下执行到没有拦截器为止，同时每一个拦截的Response是下一个拦截器执行的返回的结果

RetryAndFollowUpInterceptor最重要的是创建了StreamAllocation

3.4 BridgeInterceptor拦截器

BridgeInterceptor#intercept

 @Override public Response intercept(Chain chain) throws IOException {
    Request userRequest = chain.request();
    Request.Builder requestBuilder = userRequest.newBuilder();

    RequestBody body = userRequest.body();
    if (body != null) {
      MediaType contentType = body.contentType();
      if (contentType != null) {
        requestBuilder.header("Content-Type", contentType.toString());
      }

      long contentLength = body.contentLength();
      if (contentLength != -1) {
        requestBuilder.header("Content-Length", Long.toString(contentLength));
        requestBuilder.removeHeader("Transfer-Encoding");
      } else {
        requestBuilder.header("Transfer-Encoding", "chunked");
        requestBuilder.removeHeader("Content-Length");
      }
    }

    if (userRequest.header("Host") == null) {
      requestBuilder.header("Host", hostHeader(userRequest.url(), false));
    }

    if (userRequest.header("Connection") == null) {
      requestBuilder.header("Connection", "Keep-Alive");
    }

    // If we add an "Accept-Encoding: gzip" header field we're responsible for also decompressing
    // the transfer stream.
    boolean transparentGzip = false;
    if (userRequest.header("Accept-Encoding") == null && userRequest.header("Range") == null) {
      transparentGzip = true;
      requestBuilder.header("Accept-Encoding", "gzip");
    }

    List<Cookie> cookies = cookieJar.loadForRequest(userRequest.url());
    if (!cookies.isEmpty()) {
      requestBuilder.header("Cookie", cookieHeader(cookies));
    }

    if (userRequest.header("User-Agent") == null) {
      requestBuilder.header("User-Agent", Version.userAgent());
    }

    Response networkResponse = chain.proceed(requestBuilder.build());

    HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());

    Response.Builder responseBuilder = networkResponse.newBuilder()
        .request(userRequest);

    if (transparentGzip
        && "gzip".equalsIgnoreCase(networkResponse.header("Content-Encoding"))
        && HttpHeaders.hasBody(networkResponse)) {
      GzipSource responseBody = new GzipSource(networkResponse.body().source());
      Headers strippedHeaders = networkResponse.headers().newBuilder()
          .removeAll("Content-Encoding")
          .removeAll("Content-Length")
          .build();
      responseBuilder.headers(strippedHeaders);
      String contentType = networkResponse.header("Content-Type");
      responseBuilder.body(new RealResponseBody(contentType, -1L, Okio.buffer(responseBody)));
    }

    return responseBuilder.build();
  }

BridgeInterceptor拦截器的作用主要是添加一些网络请求的必备参数，例如Content-Type，Content-Length，Host，ConnectionAccept-Encoding，Cookie，User-Agent等，如果有使用gzip的话，还会进行gzip的处理

3.5 CacheInterceptor拦截器

3.5.1 CacheInterceptor#intercept

@Override public Response intercept(Chain chain) throws IOException {
    Response cacheCandidate = cache != null
        ? cache.get(chain.request())
        : null;

    long now = System.currentTimeMillis();

    CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
    Request networkRequest = strategy.networkRequest;
    Response cacheResponse = strategy.cacheResponse;

    if (cache != null) {
      cache.trackResponse(strategy);
    }

    if (cacheCandidate != null && cacheResponse == null) {
      closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it.
    }

    // If we're forbidden from using the network and the cache is insufficient, fail.
    if (networkRequest == null && cacheResponse == null) {
      return new Response.Builder()
          .request(chain.request())
          .protocol(Protocol.HTTP_1_1)
          .code(504)
          .message("Unsatisfiable Request (only-if-cached)")
          .body(Util.EMPTY_RESPONSE)
          .sentRequestAtMillis(-1L)
          .receivedResponseAtMillis(System.currentTimeMillis())
          .build();
    }

    // If we don't need the network, we're done.
    if (networkRequest == null) {
      return cacheResponse.newBuilder()
          .cacheResponse(stripBody(cacheResponse))
          .build();
    }

    Response networkResponse = null;
    try {
      networkResponse = chain.proceed(networkRequest);
    } finally {
      // If we're crashing on I/O or otherwise, don't leak the cache body.
      if (networkResponse == null && cacheCandidate != null) {
        closeQuietly(cacheCandidate.body());
      }
    }

    // If we have a cache response too, then we're doing a conditional get.
    if (cacheResponse != null) {
      if (networkResponse.code() == HTTP_NOT_MODIFIED) {
        Response response = cacheResponse.newBuilder()
            .headers(combine(cacheResponse.headers(), networkResponse.headers()))
            .sentRequestAtMillis(networkResponse.sentRequestAtMillis())
            .receivedResponseAtMillis(networkResponse.receivedResponseAtMillis())
            .cacheResponse(stripBody(cacheResponse))
            .networkResponse(stripBody(networkResponse))
            .build();
        networkResponse.body().close();

        // Update the cache after combining headers but before stripping the
        // Content-Encoding header (as performed by initContentStream()).
        cache.trackConditionalCacheHit();
        cache.update(cacheResponse, response);
        return response;
      } else {
        closeQuietly(cacheResponse.body());
      }
    }

    Response response = networkResponse.newBuilder()
        .cacheResponse(stripBody(cacheResponse))
        .networkResponse(stripBody(networkResponse))
        .build();

    if (cache != null) {
      if (HttpHeaders.hasBody(response) && CacheStrategy.isCacheable(response, networkRequest)) {
        // Offer this request to the cache.
        CacheRequest cacheRequest = cache.put(response);
        return cacheWritingResponse(cacheRequest, response);
      }

      if (HttpMethod.invalidatesCache(networkRequest.method())) {
        try {
          cache.remove(networkRequest);
        } catch (IOException ignored) {
          // The cache cannot be written.
        }
      }
    }

    return response;
  }

1.先从cache中以chain.request()为key获取缓存的Response，这里的request就是外部调用时创建的，这里的cache是InternalCache，实现类为okhttp3.Cache,查看get方法

3.5.2 okhttp3.Cache#get

@Nullable
    Response get(Request request) {
        String key = key(request.url());

        Snapshot snapshot;
        try {
            snapshot = this.cache.get(key);
            if (snapshot == null) {
                return null;
            }
        } catch (IOException var7) {
            return null;
        }

        Cache.Entry entry;
        try {
            entry = new Cache.Entry(snapshot.getSource(0));
        } catch (IOException var6) {
            Util.closeQuietly(snapshot);
            return null;
        }

        Response response = entry.response(snapshot);
        if (!entry.matches(request, response)) {
            Util.closeQuietly(response.body());
            return null;
        } else {
            return response;
        }
    }

1.根据请求的url，进行计算获得一个key

2.在内部cache中通过key看有没保存的快照Snapshot。这里cache是采用了DiskLruCache的算法

3.如果Snapshot不为空，通过Snapshot创建出Cache.Entry，查看一下Cache.Entry的组成

其实就是存储了一些请求返回的信息

4.通过entry.response方法获取缓存中的Response

 public Response response(Snapshot snapshot) {
            String contentType = this.responseHeaders.get("Content-Type");
            String contentLength = this.responseHeaders.get("Content-Length");
            Request cacheRequest = (new okhttp3.Request.Builder()).url(this.url).method(this.requestMethod, (RequestBody)null).headers(this.varyHeaders).build();
            return (new okhttp3.Response.Builder()).request(cacheRequest).protocol(this.protocol).code(this.code).message(this.message).headers(this.responseHeaders).body(new Cache.CacheResponseBody(snapshot, contentType, contentLength)).handshake(this.handshake).sentRequestAtMillis(this.sentRequestMillis).receivedResponseAtMillis(this.receivedResponseMillis).build();
        }

通过缓存得参数构造Request ，然后通过Request再创建出Response

5.校验缓存中的请求和相应是否和传入的Request所关联的一致

entry.matches(request, response)

6.将请求链chain中的request和缓存Response构造出一个CacheStrategy

   CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();

3.5.3 CacheStrategy#Factory方法

其实就是从缓存的cacheResponse中取出一些值进行赋值

3.5.4 CacheStrategy.Factory#get方法

 public CacheStrategy get() {
      CacheStrategy candidate = getCandidate();

      if (candidate.networkRequest != null && request.cacheControl().onlyIfCached()) {
        // We're forbidden from using the network and the cache is insufficient.
        return new CacheStrategy(null, null);
      }

      return candidate;
    }

主要逻辑在getCandidate方法中

CacheStrategy#getCandidate

 private CacheStrategy getCandidate() {
      // No cached response.
      if (cacheResponse == null) {
        return new CacheStrategy(request, null);
      }

      // Drop the cached response if it's missing a required handshake.
      if (request.isHttps() && cacheResponse.handshake() == null) {
        return new CacheStrategy(request, null);
      }

      // If this response shouldn't have been stored, it should never be used
      // as a response source. This check should be redundant as long as the
      // persistence store is well-behaved and the rules are constant.
      if (!isCacheable(cacheResponse, request)) {
        return new CacheStrategy(request, null);
      }

      CacheControl requestCaching = request.cacheControl();
      if (requestCaching.noCache() || hasConditions(request)) {
        return new CacheStrategy(request, null);
      }

      CacheControl responseCaching = cacheResponse.cacheControl();
      if (responseCaching.immutable()) {
        return new CacheStrategy(null, cacheResponse);
      }

      long ageMillis = cacheResponseAge();
      long freshMillis = computeFreshnessLifetime();

      if (requestCaching.maxAgeSeconds() != -1) {
        freshMillis = Math.min(freshMillis, SECONDS.toMillis(requestCaching.maxAgeSeconds()));
      }

      long minFreshMillis = 0;
      if (requestCaching.minFreshSeconds() != -1) {
        minFreshMillis = SECONDS.toMillis(requestCaching.minFreshSeconds());
      }

      long maxStaleMillis = 0;
      if (!responseCaching.mustRevalidate() && requestCaching.maxStaleSeconds() != -1) {
        maxStaleMillis = SECONDS.toMillis(requestCaching.maxStaleSeconds());
      }

      if (!responseCaching.noCache() && ageMillis + minFreshMillis < freshMillis + maxStaleMillis) {
        Response.Builder builder = cacheResponse.newBuilder();
        if (ageMillis + minFreshMillis >= freshMillis) {
          builder.addHeader("Warning", "110 HttpURLConnection \"Response is stale\"");
        }
        long oneDayMillis = 24 * 60 * 60 * 1000L;
        if (ageMillis > oneDayMillis && isFreshnessLifetimeHeuristic()) {
          builder.addHeader("Warning", "113 HttpURLConnection \"Heuristic expiration\"");
        }
        return new CacheStrategy(null, builder.build());
      }

      // Find a condition to add to the request. If the condition is satisfied, the response body
      // will not be transmitted.
      String conditionName;
      String conditionValue;
      if (etag != null) {
        conditionName = "If-None-Match";
        conditionValue = etag;
      } else if (lastModified != null) {
        conditionName = "If-Modified-Since";
        conditionValue = lastModifiedString;
      } else if (servedDate != null) {
        conditionName = "If-Modified-Since";
        conditionValue = servedDateString;
      } else {
        return new CacheStrategy(request, null); // No condition! Make a regular request.
      }

      Headers.Builder conditionalRequestHeaders = request.headers().newBuilder();
      Internal.instance.addLenient(conditionalRequestHeaders, conditionName, conditionValue);

      Request conditionalRequest = request.newBuilder()
          .headers(conditionalRequestHeaders.build())
          .build();
      return new CacheStrategy(conditionalRequest, cacheResponse);
    }

getCandidate方法决定CacheStrategy的构成，一般会有如下一些情况：

noCache ：不使用缓存，全部走网络

noStore ：不使用缓存，也不存储缓存

onlyIfCached ：只使用缓存

maxAge ：设置最大失效时间，失效则不使用

maxStale ：设置最大失效时间，失效则不使用

minFresh ：设置最小有效时间，失效则不使用

FORCE_NETWORK ：强制走网络

FORCE_CACHE ：强制走缓存

可以发现CacheStrategy中cacheResponse为null空有几种情况

1）没有缓存的response

2）如果这个请求是https的，但上次缓存的cacheResponse没有TLS handshake

3.通过isCacheable判断，一些请求返回值不符合要求的不缓存，还有就是请求头中有配置no-store参数时

4.请求头中声明了“no-cache”，或者“If-Modified-Since”，“If-None-Match”（服务器缓存）

5.请求头中没有添加任何条件时候

继续看 get()方法

 if (candidate.networkRequest != null && request.cacheControl().onlyIfCached()) {
        // We're forbidden from using the network and the cache is insufficient.
        return new CacheStrategy(null, null);
      }

如果外部设置了onlyIfCached（只读缓存），但缓存又无效，那就构造的CacheStrategy中既没有request也没有request

3.5.5 再次回到 CacheInterceptor#intercept

 // If we're forbidden from using the network and the cache is insufficient, fail.
    if (networkRequest == null && cacheResponse == null) {
      return new Response.Builder()
          .request(chain.request())
          .protocol(Protocol.HTTP_1_1)
          .code(504)
          .message("Unsatisfiable Request (only-if-cached)")
          .body(Util.EMPTY_RESPONSE)
          .sentRequestAtMillis(-1L)
          .receivedResponseAtMillis(System.currentTimeMillis())
          .build();
    }

1.如果设置了only-if-cached，只读缓存，但又没有缓存的Response，那就返回504

// If we don't need the network, we're done.
    if (networkRequest == null) {
      return cacheResponse.newBuilder()
          .cacheResponse(stripBody(cacheResponse))
          .build();
    }

2.networkRequest == null这里表示只用缓存，不用网络请求，那就将缓存返回

 Response networkResponse = null;
    try {
      networkResponse = chain.proceed(networkRequest);
    } finally {
      // If we're crashing on I/O or otherwise, don't leak the cache body.
      if (networkResponse == null && cacheCandidate != null) {
        closeQuietly(cacheCandidate.body());
      }
    }

3.networkRequest ！=null，那就调用接下来的拦截器进行请求，返回Response

  if (cacheResponse != null) {
      if (networkResponse.code() == HTTP_NOT_MODIFIED) {
        Response response = cacheResponse.newBuilder()
            .headers(combine(cacheResponse.headers(), networkResponse.headers()))
            .sentRequestAtMillis(networkResponse.sentRequestAtMillis())
            .receivedResponseAtMillis(networkResponse.receivedResponseAtMillis())
            .cacheResponse(stripBody(cacheResponse))
            .networkResponse(stripBody(networkResponse))
            .build();
        networkResponse.body().close();

        // Update the cache after combining headers but before stripping the
        // Content-Encoding header (as performed by initContentStream()).
        cache.trackConditionalCacheHit();
        cache.update(cacheResponse, response);
        return response;
      } else {
        closeQuietly(cacheResponse.body());
      }
    }

4.如果旧的cacheResponse不为null，又通过网络请求返回操作码304，则将新的response更新

    if (cache != null) {
      if (HttpHeaders.hasBody(response) && CacheStrategy.isCacheable(response, networkRequest)) {
        // Offer this request to the cache.
        CacheRequest cacheRequest = cache.put(response);
        return cacheWritingResponse(cacheRequest, response);
      }

      if (HttpMethod.invalidatesCache(networkRequest.method())) {
        try {
          cache.remove(networkRequest);
        } catch (IOException ignored) {
          // The cache cannot be written.
        }
      }
    }

5.新的请求进行缓存，然后过期缓存进行移除

3.6 ConnectInterceptor拦截器

ConnectInterceptor#intercept

 @Override public Response intercept(Chain chain) throws IOException {
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    Request request = realChain.request();
    StreamAllocation streamAllocation = realChain.streamAllocation();

    // We need the network to satisfy this request. Possibly for validating a conditional GET.
    boolean doExtensiveHealthChecks = !request.method().equals("GET");
    HttpCodec httpCodec = streamAllocation.newStream(client, chain, doExtensiveHealthChecks);
    RealConnection connection = streamAllocation.connection();

    return realChain.proceed(request, streamAllocation, httpCodec, connection);
  }

从链中获取出StreamAllocation，通过streamAllocation.newStream方法返回一个HttpCodec，HttpCodec的作用是对请求进行编码，然后对响应进行解码

3.6.1 StreamAllocation#newStream

  public HttpCodec newStream(
      OkHttpClient client, Interceptor.Chain chain, boolean doExtensiveHealthChecks) {
    int connectTimeout = chain.connectTimeoutMillis();
    int readTimeout = chain.readTimeoutMillis();
    int writeTimeout = chain.writeTimeoutMillis();
    int pingIntervalMillis = client.pingIntervalMillis();
    boolean connectionRetryEnabled = client.retryOnConnectionFailure();

    try {
      RealConnection resultConnection = findHealthyConnection(connectTimeout, readTimeout,
          writeTimeout, pingIntervalMillis, connectionRetryEnabled, doExtensiveHealthChecks);
      HttpCodec resultCodec = resultConnection.newCodec(client, chain, this);

      synchronized (connectionPool) {
        codec = resultCodec;
        return resultCodec;
      }
    } catch (IOException e) {
      throw new RouteException(e);
    }
  }

通过findHealthyConnection方法创建一个RealConnection，此类负责https连接的主要工作，RealConnection#newCodec方法可以创建出HttpCodec，并返回。

3.6.2 StreamAllocation#findHealthyConnection

通过findConnection方法创建RealConnection，如果这个RealConnection是全新的连接，就跳过连接健康检查，如果是之前已经连接过的RealConnection，则判断是不是一个健康的连接，如果否的话就将其从连接池connectionPool中进行回收。接下来看findConnection方法做了什么

3.6.3 StreamAllocation#findConnection

......
if (result == null) {
        // Attempt to get a connection from the pool.
        Internal.instance.get(connectionPool, address, this, null);
        if (connection != null) {
          foundPooledConnection = true;
          result = connection;
        } else {
          selectedRoute = route;
        }
      }
 ......

Internal.instance.get方法从连接池中获取是否有复用的连接，Internal是一个接口，它的实现在OkHttpClient的内部类中

1.OkHttpClient.Internal.instance#get

 @Override public RealConnection get(ConnectionPool pool, Address address,
          StreamAllocation streamAllocation, Route route) {
        return pool.get(address, streamAllocation, route);
      }

2.ConnectionPool#get

@Nullable RealConnection get(Address address, StreamAllocation streamAllocation, Route route) {
    assert (Thread.holdsLock(this));
    for (RealConnection connection : connections) {
      if (connection.isEligible(address, route)) {
        streamAllocation.acquire(connection, true);
        return connection;
      }
    }
    return null;
  }

在连接池中找出能匹配Address 的连接，注意这里的route传进来为null,connection.isEligible方法判断连接池中connection是否可复用，主要通过判断请求连接的host是否一致，具体逻辑在RealConnection#isEligible中

3.RealConnection#isEligible

public boolean isEligible(Address address, @Nullable Route route) {
    // If this connection is not accepting new streams, we're done.
    if (allocations.size() >= allocationLimit || noNewStreams) return false;

    // If the non-host fields of the address don't overlap, we're done.
    if (!Internal.instance.equalsNonHost(this.route.address(), address)) return false;

    // If the host exactly matches, we're done: this connection can carry the address.
    if (address.url().host().equals(this.route().address().url().host())) {
      return true; // This connection is a perfect match.
    }

    // At this point we don't have a hostname match. But we still be able to carry the request if
    // our connection coalescing requirements are met. See also:
    // https://hpbn.co/optimizing-application-delivery/#eliminate-domain-sharding
    // https://daniel.haxx.se/blog/2016/08/18/http2-connection-coalescing/

    // 1. This connection must be HTTP/2.
    if (http2Connection == null) return false;

    // 2. The routes must share an IP address. This requires us to have a DNS address for both
    // hosts, which only happens after route planning. We can't coalesce connections that use a
    // proxy, since proxies don't tell us the origin server's IP address.
    if (route == null) return false;
    if (route.proxy().type() != Proxy.Type.DIRECT) return false;
    if (this.route.proxy().type() != Proxy.Type.DIRECT) return false;
    if (!this.route.socketAddress().equals(route.socketAddress())) return false;

    // 3. This connection's server certificate's must cover the new host.
    if (route.address().hostnameVerifier() != OkHostnameVerifier.INSTANCE) return false;
    if (!supportsUrl(address.url())) return false;

    // 4. Certificate pinning must match the host.
    try {
      address.certificatePinner().check(address.url().host(), handshake().peerCertificates());
    } catch (SSLPeerUnverifiedException e) {
      return false;
    }

    return true; // The caller's address can be carried by this connection.
  }

通过streamAllocation.acquire(connection, true)，将connection和StreamAllocation相关联

4.StreamAllocation#acquire

 public void acquire(RealConnection connection, boolean reportedAcquired) {
    assert (Thread.holdsLock(connectionPool));
    if (this.connection != null) throw new IllegalStateException();

    this.connection = connection;
    this.reportedAcquired = reportedAcquired;
    connection.allocations.add(new StreamAllocationReference(this, callStackTrace));
  }

在这里将connection赋值给了StreamAllocation，然后connection有一个集合存放与其关联的StreamAllocation，这里StreamAllocationReference是一个弱引用。

3.6.4 再次回到ConnectInterceptor#intercept

RealConnection connection = streamAllocation.connection();
 return realChain.proceed(request, streamAllocation, httpCodec, connection);

通过streamAllocation获取出connection，继续传递给下一个拦截器

4.CallServerInterceptor 拦截器

这个是Okhttp中自带的最后一个拦截器

4.1CallServerInterceptor#intercept

@Override public Response intercept(Chain chain) throws IOException {
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    HttpCodec httpCodec = realChain.httpStream();
    StreamAllocation streamAllocation = realChain.streamAllocation();
    RealConnection connection = (RealConnection) realChain.connection();
    Request request = realChain.request();

    long sentRequestMillis = System.currentTimeMillis();

    realChain.eventListener().requestHeadersStart(realChain.call());
    httpCodec.writeRequestHeaders(request);
    realChain.eventListener().requestHeadersEnd(realChain.call(), request);

    Response.Builder responseBuilder = null;
    if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) {
      // If there's a "Expect: 100-continue" header on the request, wait for a "HTTP/1.1 100
      // Continue" response before transmitting the request body. If we don't get that, return
      // what we did get (such as a 4xx response) without ever transmitting the request body.
      if ("100-continue".equalsIgnoreCase(request.header("Expect"))) {
        httpCodec.flushRequest();
        realChain.eventListener().responseHeadersStart(realChain.call());
        responseBuilder = httpCodec.readResponseHeaders(true);
      }

      if (responseBuilder == null) {
        // Write the request body if the "Expect: 100-continue" expectation was met.
        realChain.eventListener().requestBodyStart(realChain.call());
        long contentLength = request.body().contentLength();
        CountingSink requestBodyOut =
            new CountingSink(httpCodec.createRequestBody(request, contentLength));
        BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);

        request.body().writeTo(bufferedRequestBody);
        bufferedRequestBody.close();
        realChain.eventListener()
            .requestBodyEnd(realChain.call(), requestBodyOut.successfulCount);
      } else if (!connection.isMultiplexed()) {
        // If the "Expect: 100-continue" expectation wasn't met, prevent the HTTP/1 connection
        // from being reused. Otherwise we're still obligated to transmit the request body to
        // leave the connection in a consistent state.
        streamAllocation.noNewStreams();
      }
    }

    httpCodec.finishRequest();

    if (responseBuilder == null) {
      realChain.eventListener().responseHeadersStart(realChain.call());
      responseBuilder = httpCodec.readResponseHeaders(false);
    }

    Response response = responseBuilder
        .request(request)
        .handshake(streamAllocation.connection().handshake())
        .sentRequestAtMillis(sentRequestMillis)
        .receivedResponseAtMillis(System.currentTimeMillis())
        .build();

    int code = response.code();
    if (code == 100) {
      // server sent a 100-continue even though we did not request one.
      // try again to read the actual response
      responseBuilder = httpCodec.readResponseHeaders(false);

      response = responseBuilder
              .request(request)
              .handshake(streamAllocation.connection().handshake())
              .sentRequestAtMillis(sentRequestMillis)
              .receivedResponseAtMillis(System.currentTimeMillis())
              .build();

      code = response.code();
    }

    realChain.eventListener()
            .responseHeadersEnd(realChain.call(), response);

    if (forWebSocket && code == 101) {
      // Connection is upgrading, but we need to ensure interceptors see a non-null response body.
      response = response.newBuilder()
          .body(Util.EMPTY_RESPONSE)
          .build();
    } else {
      response = response.newBuilder()
          .body(httpCodec.openResponseBody(response))
          .build();
    }

    if ("close".equalsIgnoreCase(response.request().header("Connection"))
        || "close".equalsIgnoreCase(response.header("Connection"))) {
      streamAllocation.noNewStreams();
    }

    if ((code == 204 || code == 205) && response.body().contentLength() > 0) {
      throw new ProtocolException(
          "HTTP " + code + " had non-zero Content-Length: " + response.body().contentLength());
    }

    return response;
  }

这个拦截器主要的任务如下：

1.写入请求头

2.写入请求体

3.读取响应头

4.读取响应体

这样所有的Okhttp流程基本分析完毕，再往深入网络连接流这一块能力有限无法进行分析

本文参与腾讯云自媒体同步曝光计划，分享自作者个人站点/博客。

原始发表：2019-06-26，如有侵权请联系 cloudcommunity@tencent.com 删除

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